The HYSUHULL principle can be applied to any ship hull, monohull, catamaran hulls or multi-hull vessel. A special advantage is gained when the principle is applied to catamaran hulls, in short called HYSUCAT. Here it allows a most suitable and compact ship construction. The foil is arranged in the gap between the two demihulls, and is well protected by the hulls. The strength of the catamaran is increased by the foils near the keel line forming a ringlike frame structure connecting the two demihulls to each other. The deck covers the two demihulls and the tunnel gap. The necessary foil area to carry a portion of the ship weight (eg. 50%) is dependent, in addition to the other parameters involved, on the square of the ship speed (V.sup.2). Therefore for high speed ships it is much smaller than for low speed hulls. This influences the tunnel width. In speed ranges of usual planing craft, the necessary tunnel width is relatively small allowing HYSUCAT designs with similar hull proportions as deep-V-planing craft. The high speed catamaran is best equipped with fully asymmetrical demihulls, which allow a straight tunnel with parallel vertical side walls and low flow interference effects inside the tunnel.
The nval architect uses a dimensionless speed, defined as the Froude displacement number ##EQU1## with V=ship speed, g=acceleration of earth, .gradient.=displaced volume.
For very low F.sub.n.gradient. the usual displacement hull is the most efficient (say up to F.sub.n.gradient. =2,3) but has to be built extremely slender for the higher speeds to be efficient. The HYSUCAT is more efficient for F.sub.n.gradient. .gtoreq.2,3 compared with mono displacement hulls and planing deep-V-craft. When compared with a planing-deep-V-craft, the HYSUCAT is more efficient from F.sub.n.gradient. .gtoreq.1,6 under the consideration of a compact structure (excluding extreme L/B proportions).
For HYSUCAT craft designed for the lower speeds, but F.sub.n.gradient. .gtoreq.1,6, the use of partly asymmetrical demihulls or symmetrical demihulls may be advantageous, however for much lower F.sub.n.gradient. values the necessary support-hydrofoil area will have to be relatively large and a part of the resistance gain is lost in higher friction resistance over the foil. The HYSUCAT cannot improve on the low speed displacement hull, especially if it is a comparable monohull.
The HYSUCAT therefore has primarily to be considered as a "High Speed Small Craft". None of the prior patents or patent applications known to the applicant resulted in the building of a practical HYSUCAT craft. This is mainly due to the fact that none of the proposed designs allowed the combination of the hull and support hydrofoils without negative interference of each other, which results in either a high resistance or insufficient trim and transverse stability especially at speed.
According to the invention, a catamaran type boat is provided having two similar boat demi-hulls which are spaced apart and which are substantially parallel, each demi-hull having a base line (BL) extending longitudinally tangentially to the lowest boundary of the surface of the demi-hull at the midship ordinate, the boat further including
(a) a superstructure conecting the two demi-hulls transversely;
(b) an open space in the form of a tunnel defined between the superstructure and the two demi-hulls;
(c) a longitudinal centre of gravity position (LCG) for the boat;
(d) at least one main hydrofoil, having a cord line (CL) extending between its leading edge and trailing edge and extending at least partially across the tunnel, and being adapted to be under water;
(e) at least one trim hydrofoil having a chord line (CL) extending between its leading edge and trailing edge and being located in the stern region of the boat and extending at least partially across the tunnel; the projected area of the main hydrofoil being larger than the combined projected area of all trim hydrofoils; and
(f) attachment means for attaching all hydrofoils to the demi-hulls substantially along a transverse plane (TP), which is substantially at right angles to the longitudinal vertical centre plane of the boat, and having an angle of between 1.degree. and 7.degree. to the base line (BL) of the demi-hulls at the main foil, and with the hydrofoil chord lines (CL) being at an angle of between 0.degree. and 6.degree. to the transverse plane (TP), the attachment means being adapted to locate the hydrofoils such that their combined resultant lift-force at speed is adapted to act lengthwise through a point in the vicinity of the longitudinal centre of gravity (LCG).
The main hydrofoil may be located substantially in the vicinity of the LCG of the boat, and the superstructure may be adapted to be above water when the boat is at speed.
The attachment means may locate each hydrofoil such that at highest speed the average water height (HW) over it is less than the main hydrofoil chord length (CL) and preferably being 20% to 50% thereof.
The projected area of the main hydrofoil may be about 3 to 5 times larger than the combined projected area of all trim hydrofoils.
The hydrofoils in the transverse plane may have a leading edge which has a slight dihedral angle.
The hydrofoils may be built up of subcavitating foil-profile-sections with a circular upper surface and a flat lower surface and a rounded leading edge, or the hydrofoils may be built up of supercavitating foil-profile-sections with wedge-like shape, sharp leading edge and blunt trailing edge.
The demi-hulls may be of the fully assymetrical planing hull type, preferably with deep-V planing hull characteristics.
The demi-hull side walls facing towards each other may be substantially flat and substantially straight forming a substantially straight tunnel in flow direction with about vertical parallel tunnel side walls.
The attachment means may attach each main hydrofoil near and slightly above the base line (BL) of each demi-hull.
The superstructure connecting the two demi-hulls may include a tunnel ceiling, which is watertight and which is located at a position to come into water contact when the boat is at rest or moves at low speeds.
At least one main hydrofoil may extend fully across the tunnel and at least one pair of trim hydrofoils extends partially across the tunnel.
Height adjustment means may be provided for adjustment of the height of the trim foil(s) over keel, in order to adjust or change the trim-angle of the boat at speed.
Angle adjustment means may be provided to adjust the angle of attack of the foils towards the hulls.
The invention therefore attempts to improve on the disadvantages of prior hull foil arrangements and purposes a double foil arrangement, which has self-trimming characteristics. Therefore the positioning of LCP (longitudinal centre of pressure of foil) in relation to LCG (longitudinal centre of gravity of craft) is less critical. The boat therefore should function properly and efficiently in the full range of all practically possible LCG positions, which is especially important for the smaller and less complicated boats. To achieve this at least two support hydrofoils must be arranged in such way as to contribute to the longitudinal stability of the craft. This is possible, in accordance with the invention, by fitting the foils to the hull in such positions that the foils operate at design speed in surface nearness, which results in reduced lift forces. The foil's lift in the so-called "surface effect" is dependent on a further parameter, the height of water over the foil (HW) in relation to the foil's chord length (CL). The foil starts to "feel" the surface when the ratio HW/CL=1,0. For smaller ratios HW/CL the lift force falls off and it reaches a value of about 50% when the foil's leading edge breaks the surface. The lift forces reduce with further emergence until they are zero when the foil's trailing edge leaves the water (the second stage is similar to planing).
From the test results it followed that the foil should be operational at design speed in the surface effect for values of HW/CL=0,15 to 0,4 with the lift force being reduced to about 32% to 54% of its value when fully submerged. However, operation is also possible when the top surface of the one or the other foil, or both foils, is free of water contact and the lower foil surface acts like a planing surface. At this stage the efficiency is reduced but at high speeds it may be acceptable.
A support hydrofoil designed to operate in the surface effect mode in the tunnel of the catamaran boat under discussion brings the advantage that the foil with a specific load shows the tendency to run at a constant level of submergence. If it is pushed deeper into the water, it will develop strongly increasing lift-forces, which tend to bring it back to the design level. The increase of the lift forces, due to surface effect, are independent of the attack angle. The foil surface effect gives therefore an alternative way of regulating the lift forces without changing the trim angle of the boat, simply by submergence and this can be used to stabilise the boat longitudinally and also partly transversely.
As stated above in order to achieve this objective, in accordance with the invention, the catamaran has at least two foils fitted, a main foil slightly in front of LCG and a trim foil (or foil pair) behind the LCG near to the stern. The foils can have different projected areas, and different distances to the LCG position of the craft, for example a special advantage is reached with a larger main foil slightly in front of LCG and a small trim foil near the stern of the craft. The resultant lift force of all foils in the design and optimum conditions must act through a centre of pressure near the LCG to allow a favourable trim of the boat at all speeds. It is not practical to fit the main foil too far ahead of the LCG position (say near the bows) as it would "suffer" the strongest trim motions there, and leave the water in waves periodically with hard impact motions. It therefore is advantageous to have the main foil as near as possible to the LCG position, so that the trim balance can be maintained at all speeds. This means that the trim foil (or foil pair) with the longer distance to the LCG position has to be as small as possible but large enough to fulfill the trim balancing job, which depends on the LCG shifts to be expected during operation. On a smaller boat the LCG shift is stronger and relatively larger trim foils will be required which "force" the main foil positioning forwardly so that the resultant lift force of all foils acts at or near the LCG position.
Further, if the foil would penetrate deeper than the lateral area of the hull, this would render the foil vulnerable to contact with floating objects, structures at sea bottom or harbour installations. It is rather more favourable to have the foils inside the protected tunnel space.
The foils must be dimensioned to carry the part load (they are supposed to take) of the craft's weight in the surface effect mode. This means, their areas must be, based on the above explanations, larger by the factor of the lift reduction in surface effect for design speed. They must then be fitted in the inside of the tunnel of the two demihulls in such a way that they come at design speed in the desired surface effect position and in the meantime allow the demihulls to run partly submerged with a favourable or optimum trim angle .psi.. It means the foils must be fitted to the demihulls in a way that they have about the same depth of submergence (preferably HW/CL=0,15 to 0,4) when the hull is running with a favourable or optimal trim at design speed. The forward foil is positioned deeper towards the demi-hull keel and in most cases just slightly above keel height whereas the stern foils are situated higher above the keel line. The foils operate in the surface effect mode and have about optimal attack angles .alpha..sub.1,2 towards the inflow, the hull has a favourable or optimal trim angle .psi.. In this way the resistance improvement is optimal.
The invention will now be described by way of example with reference to the accompanying schematic drawings.